The first reference to 10000's designation as Class W1 was on 9 November 1926, but where the widely adopted name of 'Hush-Hush' came from is disputed. It could be that the nickname arose in the late summer of 1928, as news of Gresley's new locomotive began to leak out, however other sources suggest this was possibly an internal Darlington name that came from the distinctively quiet motion sound the loco made, or even because of a mistaken caption that featured in a Pathé News item. On 20 November 1929, nameplates were drawn up featuring the name British Enterprise and were cast soon after, but these plates were never fitted and the loco remained unnamed, being referred to as 'Hush-Hush' ever since.
Yarrow's had completed the boiler in February 1929 and on April 3rd the chassis was moved from Darlington to Glasgow for testing, these being completed by October 1929 and it was during this period that wind tunnel tests led to the unusual front-end design that gave the W1 part of its distinctive appearance. Official photographs were released on December 10th and the first trial run took place on December 12th, followed by another on December 29th and, on 9 January 1930, the first trip to Kings Cross, although it was to be a further six months of trials though before 10000 finally entered traffic on June 20, 1930. Between June 1930 and May 29, 1933, 10000 ran 70,000 miles in service, from Kings Cross to Edinburgh, including the Flying Scotsman and Queens of Scots services, but in 1934 modifications and a general repair took 10000 out of service for just over a year, the locomotive returning to traffic on June 13, 1934.